There are at least two proposed isoforms of the human progesterone receptor (PR) and PRa inhibits the transcriptional activity of PRb. If true, functional progesterone withdrawal could occur in gestational tissues while total PR remains unchanged. To investigate this hypothesis, we used a Western Immunoblot system to determine if PRa and PRb are present in gestational tissues of rhesus monkeys. Myometrium and decidua were collected from pregnant monkeys at term by cesarean section and immediately frozen in liquid nitrogen. Extracted protein was loaded onto SDS-polyacrylamide gels and electrophoresed. Proteins were transferred to nitrocellulose membranes and exposed to one of several human PR antibodies (JZB39 [G.L. Greene], C262, AB52 [K.B. Horowitz], Immunotech Iopath) to determine specificity and cross-reactivity. Blots were exposed to autoradiograph film, scanned into a computer, analyzed by computer densitometry and expressed in relative image intensity units. T47D cells were used as a PR-positive control and monkey spleen as a PR-negative control. Bands for monkey PRa and PRb migrated to the same molecular size positions as T47D cells. Band intensity for decidua and myometrium increased with added protein. Intensity of signal for PRa and PRb was similar in T47D cells and consistent between gels. Signals obtained from myometrial tissues also showed similar intensities for PRa and PRb. Band intensity was significantly less for decidual PRb when compared to PRa. Band Intensities from Western Immunoblot Using JZB39 (*= P<0.05) PRa PRb Decidua (n=4) 218.8 q 4.3 166.3 q 8.0* Myometrium (n=4) 216.3 q 9.9 217.5 q 9.2 T47D Cells (n=6) 245.0 q 2.2 243.2 q 3.3 This is the first demonstration of PR isoforms in gestational tissues from rhesus monkeys. The observation that PRa is greater than PRb in decidua may have functional consequences in the transition from pregnant to parturient physiologic states in non-human primates.